Tightening of threaded fasteners to the proper load is of great importance. Torque wrenches which have been widely used for many years for controlling fastener loads suffer the disadvantages of relying on torquetension relationship which varies with friction due to lubrication for example and results in questionable accuracy of load measurements.
Another type of bolt load determining apparatus involves tightening the bolt to its yield point. Most such units are large and are part of permanent installations. Tightening a bolt to its yield point may be undesirable in many situations.
Prior systems generally suffer the disadvantages of need for special fasteners components, extensive time to make the measurements, special access for clearances, and expensive power equipment.
In response to these undesirable features of available bolt load testing systems then on the marketplace Dr. Joseph Heyman devised an ultrasonic instrument described in his U.S. Pat. No. 4,117,731.
The patented system includes a transducer which is affixed to one end of the bolt under test. Signals are produced at a harmonic of the resonant frequency of the bolt. The instrument produces signals almost continuously (99.8% of the time). But, for a short part of each cycle (0.2%), the system evaluates the response of the bolt to the signal. A bolt has a bell-shaped response curve when frequency is plotted against energy. The peak in energy represents the frequency at which the bolt is resonant. The frequency is fairly low and is in the audible range. However, there are minor peaks representing resonance at harmonics of the resonant frequency. Dr. Heyman determined that if high frequencies are selected, a node for most bolts will fall within a selected range such as 4.8-5.2 mHz. A given bolt may be resonant at say 5 mHz which is, for example, the 127th node.
The patent describes adjusting the variable phase shifter 27 in the Heyman instrument until the output is maximized. The frequency of resonance is displayed on the readout 34. First, the frequency of resonance is determined for the bolt in its unloaded condition. When the bolt is thereafter tightened, the frequency of the resonant peak will shift, as described in the patent, and the new frequency of resonance will be continuously displayed on the readout 34.
Basically, the Heyman instrument causes the frequency applied to the bolt to always be such that it is at the same node. In other words, if in making the initial adjustment, the 127th node is selected, and the bolt is then stressed to cause its resonant frequency to change, the frequency applied to the bolt will not only be such as to place it in resonance, but also to place it in resonance at the same node.
The difficulty with the Heyman instrument is that the frequency of resonance of the unstressed bolt and/or the stressed bolt is not useful in and of itself. The frequency must be translated to load on the bolt.